Deep-well developments of oil wells and gas wells (oil wells and gas wells are collectively referred to simply as “oil wells”, hereinafter) require high strength of oil-well steel pipes. Conventionally, 80 ksi-grade (yield stress of 80 to 95 ksi, that is, 551 to 654 MPa) and 95 ksi-grade (yield stress of 95 to 110 ksi, that is, 654 to 758 MPa) oil-well steel pipes have been widely used. However, 110 ksi-grade (yield stress of 110 to 125 ksi, that is, 758 to 862 MPa) oil-well steel pipes have recently come into use.
Most deep-wells contain hydrogen sulfide having corrosiveness. Hence, oil-well steel pipes for use in deep wells are required to have not only a high strength but also a sulfide stress cracking resistance (referred to as a SSC resistance, hereinafter). In general, susceptibility to the SSC is increased along with increase in strength of a steel material.
Steel pipes of 110 ksi grade or less sold as sour-resistant oil-well steel pipes (sour service OCTG) usually have a guaranteed SSC resistance. The guaranteed SSC resistance herein denotes durability performance under the H2S environment at 1 atm in an evaluation in a test method specified by NACE. Hereinafter, the H2S environment at 1 atm is referred to as a standard condition.
Meanwhile, a SSC resistance guaranteed for oil-well steel pipes of 125 ksi grade (yield stress of 862 to 965 MPa) is smaller than the above SSC resistance. In these oil-well pipes, only the SSC resistance under an environment in which partial pressure of H2S is much smaller than that under the standard condition is guaranteed in many cases. This means that, once the lower limit of the yield strength becomes more than 110 ksi (758 MPa), it becomes suddenly difficult to secure an excellent SSC resistance.
On this background, there is a need for sour-resistant oil-well steel pipes that secures the SSC resistance under the H2S environment at 1 atm, and has a yield strength as great as possible. In this case, even if the lower limit of the yield strength does not reach 125 ksi (862 MPa), the lower limit of the yield strength is required to be as great as possible.
Techniques to enhance the SSC resistance of oil-well steel pipes are disclosed in Japanese Patent Application Publication No. 62-253720 (Patent Literature 1), Japanese Patent Application Publication No. 59-232220 (Patent Literature 2), Japanese Patent Application Publication No. 6-322478 (Patent Literature 3), Japanese Patent Application Publication No. 8-311551 (Patent Literature 4), Japanese Patent Application Publication No. 2000-256783 (Patent Literature 5), Japanese Patent Application Publication No. 2000-297344 (Patent Literature 6), Japanese Patent Application Publication No. 2005-350754 (Patent Literature 7), National Publication of International Patent Application No. 2012-519238 (Patent Literature 8), Japanese Patent Application Publication No. 2012-26030 (Patent Literature 9), and International Application Publication No. WO2010/150915 (Patent Literature 10).
Patent Literature 1 proposes a method of enhancing the SSC resistance of an oil-well steel pipe by reducing impurities such as Mn and P. Patent Literature 2 proposes a method of enhancing the SSC resistance of steel by performing quenching twice to refine grains.
Patent Literature 3 proposes a method of enhancing the SSC resistance of a 125 ksi-grade steel material by refining steel microstructure through an induction heat treatment. Patent Literature 4 proposes a method of enhancing the SSC resistance of a steel pipe of 110 ksi grade to 140 ksi grade by enhancing hardenability of the steel through direct quenching process, and increasing a tempering temperature.
Each of Patent Literature 5 and Patent Literature 6 proposes a method of enhancing the SSC resistance of a low alloy oil-well steel pipe of 110 ksi grade to 140 ksi grade by controlling the morphology of carbide. Patent Literature 7 proposes a method of enhancing the SSC resistance of an oil-well steel pipe of 125 ksi (862 MPa) grade or more by controlling a dislocation density and a hydrogen diffusion coefficient to be desired values. Patent Literature 8 proposes a method of enhancing the SSC resistance of 125 ksi (862 MPa)-grade steel by quenching low alloy steel containing C of 0.3 to 0.5% several times. Patent Literature 9 proposes a method of employing a two-stage tempering step to control the morphology of carbide and the number of carbide particles. More specifically, in Patent Literature 9, the SSC resistance of 125 ksi (862 MPa)-grade steel is enhanced by suppressing the number density of large M3C particles or WC particles. Patent Literature 10 proposes a method of coping with both a high strength and the SSC resistance by controlling amount of dissolved Mo, a prior-austenite grain size, and amount of WC precipitate to be desired values.
However, even if applying the techniques disclosed in the above Patent Literatures 1 to 10, in the case of oil-well steel pipes having a yield strength 120 ksi (827 MPa) or more, an excellent SSC resistance cannot be stably obtained in some cases.